Much of the software associated with Bitcoin is open source, i.e. wallets, address generators, mining software, operating systems... Open source means the raw computer code is publicly available for scrutiny. Open source software is usually considered less buggy and more secure.
One common obstacle with open source software is transforming it from the raw computer code to a program that runs on your phone, tablet, laptop, desktop. This process is called 'building from source.
Download Here: https://github.com/bitcoin/bitcoin/commit/bd34174ebca239e6796f0eb2015ddc2f218aac3c
